1. Field of the Invention
The present invention relates to a ceramic electronic device and to a paste coating method and paste coating apparatus used for production of that ceramic electronic device.
2. Description of the Related Art
In chip capacitors and other ceramic electronic devices, cap-shaped terminal electrodes are formed at the two end faces of the ceramic body. More particularly, each of these cap shaped terminal electrodes is formed to cover one end face of the ceramic body in the length direction, parts of the two surfaces of the body in the width direction, and parts of the two surfaces in the thickness direction.
To form such terminal electrodes, the general practice has been to use the method of dipping the ceramic body in a conductor paste or coating by a roller to coat a conductive paste on the two ends of the ceramic body and then dry the conductive paste by heat treatment to solidify it.
Terminal electrodes formed by conductive paste sometimes differ in the lengths of the parts formed on one surface in the width direction and the lengths of the parts formed on the other surface in the width direction due to the conditions under which the conductive paste is coated.
If soldering on such a ceramic electronic device, the contact areas between the terminal electrode and solder at the two surfaces in the width direction will end up differing. Therefore, the terminal electrode will receive stress of different magnitudes from the solder and the inconvenience of the phenomenon of the ceramic body standing up on its own (Manhattan phenomenon) etc. will arise.
An object of the present invention is to provide a ceramic electronic device able to avoid the Manhattan phenomenon of the ceramic body in the soldering process and a paste coating method and paste coating apparatus able to be used for production of such a ceramic electronic device.
To achieve the above object, according to a first object of the present invention, there is provided a ceramic electronic device having a ceramic body and terminal electrodes, wherein the ceramic body is substantially a rectangular parallelopiped in shape; the electrode terminals are provided at the two end sides of the ceramic body in the length direction, each of the terminal electrodes provided to cover one end face in the length direction of the ceramic body, part of the two surfaces in the width direction, and part of the two surfaces in the thickness direction; and, when the length of the ceramic body is L1 and the maximum lengths of the terminal electrodes at the two surfaces of the ceramic body in the width direction are L3 and L4, 0xe2x89xa6|L4xe2x88x92L3|/L1xe2x89xa60.0227 (1/44) is satisfied.
Experiments of the present inventors revealed that if the ratio (|L4xe2x88x92L3|/L1) is set in a range from 0 to 0.0227, the rate of occurrence of the Manhattan phenomenon in the soldering process is kept low. Over the ratio |L4xe2x88x92L3|/L1=0.0227, the rate of occurrence of the Manhattan phenomenon tends to abruptly increase. Therefore, according to the ceramic electronic device of the present invention, it is possible to effectively suppress the Manhattan phenomenon and eliminate the phenomenon of the ceramic body standing up in the soldering process.
Preferably, the ceramic body has a plurality of internal electrodes, the plurality of internal electrodes being stacked inside the ceramic body in the thickness direction at intervals from each other and being connected to the terminal electrodes at one end surface each in the length direction. The effect of the present invention is particularly great in the case of a ceramic electronic device having such a ceramic body.
Preferably, each of the terminal electrodes is formed by coating a conductive paste on one end face of the ceramic body in the length direction by roller coating and heat treating it.
In this case, preferably one surface among the two surfaces of the ceramic body in the width direction is a paste introduction side in the roller coating, while the other surface is the paste escape side in the roller coating.
Preferably, the boundary shapes of the terminal electrodes formed at parts of the two surfaces of the ceramic body in the width direction are different. More preferably, the boundary shapes of the terminal electrodes formed at parts of the two surfaces of the ceramic body in the width direction are different, with one being a projecting type shape and the other being a recessed type shape. Alternatively, the boundary shapes of the terminal electrodes formed at parts of the two surfaces of the ceramic body in the thickness direction are inclined with respect to the end faces of the ceramic body in the length direction.
When forming a terminal electrode on one end face of the ceramic body in the length direction by roller coating, the boundary shape with the body of the terminal electrode easily becomes this shape. In this case, by applying the present invention, it is possible to effectively suppress the Manhattan phenomenon.
The surface of the terminal electrode may be formed with a plating film. This plating film may be a single layer or multiple layers.
According to a second aspect of the present invention, there is provided a paste coating method for forming terminal electrodes comprising the steps of rotating a roller coated with a conductive paste on its outer circumference and moving a ceramic body in substantially the same direction as the direction of rotation of the roller and bringing one end surface of the ceramic body into contact with the conductive paste present on the outer circumference of the roller to coat it with the conductive paste, wherein, when the speed of movement of the ceramic body is Vc and the peripheral speed of the roller at the circumference in contact with the one end surface of the ceramic body is Vp, the ratio Vp/Vc satisfies 0.95xe2x89xa6Vp/Vcxe2x89xa60.98.
In this case, the ceramic body preferably has a plurality of internal electrodes, the plurality of internal electrodes being stacked inside the ceramic body at intervals from each other and being exposed at one end surface each of the ceramic body.
Experiments of the present inventors confirmed that by setting the ratio Vp/Vc to a range of 0.95 to 0.98 and executing the paste coating method, it is possible to keep the ratio (|L4xe2x88x92L3|/L1) of the terminal electrodes formed in the range from 0 to 0.0227. If setting the ratio Vp/Vc larger than 0.98, the ratio (|L4xe2x88x92L3|/L1) of the terminal electrodes exceeds 0.0227. Further, even if setting the ratio Vp/Vc smaller than 0.95, the ratio (|L4xe2x88x92L3|/L1) of the terminal electrodes exceeds 0.0227.
By using the paste coating method of the present invention, it is possible to effectively produce the above-mentioned ceramic electronic device resistant to the Manhattan phenomenon.
According to a third aspect of the present invention, there is provided a paste coating apparatus including a paste coating roller for coating a paste for forming terminal electrodes and a ceramic body movement device, the roller being driven to rotate; the ceramic body movement device moving the ceramic body in substantially the same direction as the direction of rotation of the roller and bringing one end surface of the ceramic body into contact with paste present on the outer circumference of the roller; and, when the speed of movement of the ceramic body is Vc and the peripheral speed of the roller at the circumference in contact with the one surface of the ceramic body is Vp, the ratio Vp/Vc satisfies 0.95xe2x89xa6Vp/Vcxe2x89xa60.98.
This paste coating apparatus is used to coat a conductive paste on the outer circumference of the roller. It rotates the roller in this state and moves the ceramic body in substantially the same direction as the rotational direction of the roller. Further, it brings one end surface of the ceramic body into proximity with the outer circumference of the roller. Due to this, the conductive paste on the outer circumference of the roller is coated on that surface of the ceramic body. Therefore, the above paste coating method is realized and the above-mentioned ceramic electronic device according to the present invention is obtained.
Preferably, in the latter position of the direction of movement of the ceramic body with respect to the paste coating roller, a scrape-off roller is placed rotating in substantially the opposite direction to the direction of movement of the ceramic body for controlling the thickness of the paste coated on the one surface of the ceramic body.
By providing this scrape-off roller, it is possible to form a terminal electrode of a uniform predetermined thickness on one end surface of the ceramic body.